Sister María Gómez has been formally named as a suspect in the investigation into one of more than 1,500 cases of suspected illegal trafficking of babies who were stolen, sold or given away by adoption over four decades until the 1980s.

Although the nun's name appears in many of the complaints made by those seeking lost children, the accusation centres on a single case involving María Luisa Torres and her 29-year-old daughter, Pilar. They were reunited eight months ago after families affected by the trafficking began campaigning for the truth.

Gómez has reportedly refused to give investigators her version of how Pilar was adopted in 1982.

Torres told the magistrate she had contacted Gómez after separating from her husband and becoming pregnant by another man. The nun, who worked with two Madrid clinics in the 1970s and 80s, had placed ads in local newspapers offering to help single mothers. Torres said Gómez promised to put the baby in an orphanage where she could visit her until she was able to look after the child herself. But after the birth Gómez told her the baby had been given to another family.

When Torres complained, the nun allegedly threatened to denounce her for the crime of infidelity and have her other daughter, born to her first husband, taken away.

"Her words were: 'I'm taking this one away and I can take the other one too. And then you'll go to jail,'" Torres told Antena 3 television.

Pilar's adopted parents took her to see Gómez when she began to ask after her birth mother. The nun allegedly told them Pilar's mother had been a prostitute.

Last year, as hundreds of alleged cases of baby snatching began to emerge, Torres's other two daughters set out to find their lost sister. In July a TV programme put the families in contact and paid for DNA tests.

"Finding my daughter means I can finally rest in peace," Torres said.

Both the justice ministry and the attorney general's office have said that resolving the stolen babies scandal is a priority, but victims complain most cases are eventually shelved. In at least three cases that remain open, graves have been opened and found to be empty.

Torres told El País newspaper that her case had only been resolved "because I was able to go to the prosecutor's office hand-in-hand with my daughter".

Campaigners are demanding a government inquiry into the scandal. They claim that networks of doctors, nurses and nuns who stole and sometimes sold babies were allowed to operate freely in many parts of Spain up until the early 1980s.

A campaign that started with an alleged 261 victims in January 2011 has since seen more than 1,500 complaints lodged at court houses around the country.

It appears that what began as a system for removing babies from families deemed politically dangerous by the dictatorship of General Francisco Franco allegedly evolved into an illicit business that targeted single mothers and the poor.

In one Madrid clinic where Gómez worked it is alleged that doctors kept a baby's corpse to show to mothers as proof that their child had died.